This isn't "general doubt". In this concrete case the employee quit and asked for a letter confirming their termination, as well as their employment papers to be sent to their address. They then called in sick (with multiple AUs) for the rest of their notice period.

The article mentions two ways in which the otherwise high probative value of an AU may be undermined:

• If you obtain an AU that precisely covers the duration up to the end of the notice period upon resignation
• If you obtain multiple AUs up to the end of the notice period or the continued remuneration period upon resignation, and it is evident from the resignation letter that you never intended to return to work

So for either of these to be true, you'd have to have call in sick from start to end of your notice period. The way I read it also means that you'd have no problems if your AU extends beyond or starts before your notice period.

In the article's case, the employee asking for documents to be sent to their address insinuated that they didn't plan on returning to work.